Add callback for GIF rendering completion

[selalipop]

Currently createPartyImage returns before the image has fully rendered, this can cause invalid image output when the outputStream is immediately read, especially for larger input images, such as animated GIFs

These changes add a callback which is fired when the gif finishes rendering, as defined by the end event

[selalipop]

~~Is it just me or is the indentation not right in the GitHub preview?~~

~~Locally I don't see that indentation in Sublime Merge or VSCode...~~

(fixed somehow 🎉 )

[scotchfield]

Do you have a reproducible case where this occurs? I'm not entirely sure what problem this is fixing. Happy to consider the change! Sorry for not looking at this PR sooner. :(